1. Field of the Invention
The invention relates to a transmitter and an adjusting method thereof, more particularly to a transmitter capable of reducing local oscillation leakage and in-phase/quadrature-phase (I/Q) mismatch, and an adjusting method thereof.
2. Description of the Related Art
As shown in FIG. 1, a first conventional direct up-conversion transmitter includes first and second digital-to-analog converters 11, 12, first and second low-pass filters 13, 14, first and second mixers 15, 16, an adder 17, a power amplifier 18, and an antenna 19. A digital base band signal (BBIt) undergoes in sequence digital-to-analog conversion by the first digital-to-analog converter 11, low-pass filtering by the first low-pass filter 13, and mixing with an in-phase local oscillator signal (LOIt) by the first mixer 15 so as to generate an analog in-phase radio frequency signal (RFIt). Another digital base band signal (BBQt) undergoes in sequence digital-to-analog conversion by the second digital-to-analog converter 12, low-pass filtering by the second low-pass filter 14, and mixing with a quadrature-phase local oscillator signal (LOQt) by the second mixer 16 so as to generate an analog quadrature-phase radio frequency signal (RFQt). The analog in-phase radio frequency signal (RFIt) and the analog quadrature-phase radio frequency signal (RFQt) are combined by the adder 17, the result of which is amplified by the power amplifier 18 for subsequent transmission via the antenna 19.
Although the ideal phase difference between the in-phase local oscillator signal (LOIt) and the quadrature-phase local oscillator signal (LOQt) is 90 degrees, a phase offset (θt) exists in practice. In addition, a gain offset (represented by an amplitude offset (αt) in FIG. 1) exists between the in-phase component blocks (including the first digital-to-analog converter 11 and the first low-pass filter 13) and the quadrature-phase component blocks (including the second digital-to-analog converter 12 and the second low-pass filter 14). This phenomenon is referred to as in-phase/quadrature-phase (I/Q) mismatch or in-phase/quadrature-phase (I/Q) imbalance. Moreover, it is possible for the in-phase local oscillator signal (LOIt) and the quadrature-phase local oscillator signal (LOQt) to respectively leak into the analog in-phase radio frequency signal (RFIt) and the analog quadrature-phase radio frequency signal (RFQt) through the first and second mixers 15, 16, respectively. This phenomenon is called local oscillation leakage or local oscillation feedthrough. The abovementioned I/Q mismatch and local oscillation leakage reduce signal-to-noise ratio of signals transmitted by the first conventional direct up-conversion transmitter, and eventually result in loss of data.
As shown in FIG. 2, U.S. Pat. No. 6,970,689 discloses a second conventional transmitter capable of reducing local oscillation leakage. The second conventional transmitter includes a mixer 21, a power amplifier 22, a signal strength measuring circuit 23, and a control signal generating circuit 24. The mixer 21 is operable is in a plurality of operating states that respectively correspond to various extents of local oscillation leakage. The signal strength measuring circuit 23 is used for measuring signal strength of a local oscillation leakage component of an output signal outputted by the power amplifier 22. The signal strength measuring circuit 23 includes a rectifier (not shown) and a comparator (not shown). The control signal generating circuit 24 outputs a control signal to control the operating state of the mixer 21 according to output of the signal strength measuring circuit 23.
During calibration of the mixer 21, for each possible operating state of the mixer 21, the power amplifier 22 operates at a higher gain level, and the control signal generating circuit 24 stores information related to the operating state and the corresponding signal strength measured by the signal strength measuring circuit 23. To complete the calibration process, the operating state corresponding to the lowest extent of local oscillation leakage is selected as a current operating state for the mixer 21.
Alternatively, during calibration of the mixer 21, the power amplifier 22 operates at a higher gain level, and the control signal generating circuit 24 changes the current operating state of the mixer 21 in succession, until the corresponding signal strength measured by the signal strength measuring circuit 23 is smaller than a preset threshold value, at which time the current operating state of the mixer 21 is fixed.
FIG. 3 illustrates a third conventional transmitter capable of reducing local oscillation leakage and I/Q mismatch as disclosed by C. Lee et al., “A Highly Linear Direct-Conversion Transmit Mixer Transconductance Stage with Local Oscillation Feedthrough and I/Q Imbalance Cancellation Scheme”, Solid-State Circuits, 2006 IEEE International Conference Digest of Technical Papers (San Francisco, U.S.A.), pp. 1450-1459, Feb. 6-9, 2006. The third conventional transmitter includes first and second digital-to-analog converters 301, 302, first and second low-pass filters 303, 304, first and second transconductance stages 305, 306, first and second mixers 307, 308, an adder 309, a power amplifier 310, an antenna 311, an envelope detector 312, and a variable gain amplifier 313. First and second digital base band signals (BBIt), (BBQt) are respectively converted into first and second analog radio frequency signals (RFIt), (RFQt), which are combined and amplified for subsequent transmission.
The envelope detector 312 and the variable gain amplifier 313 sequentially perform envelope detection and amplification upon an output signal of the power amplifier 310 so as to generate a base band ripple. When the first and second digital base band signals (BBIt), (BBQt) are sinusoidal signals with frequencies of (FBB), spectral components of the base band ripple appear at (FBB) (due to local oscillation leakage) and (2×FBB) (due to I/Q mismatch). Therefore, spectral analysis of the base band ripple reveals the extents of local oscillation leakage and I/Q mismatch.
Local oscillation leakage can be categorized into base band local oscillation leakage and radio frequency local oscillation leakage. Base band local oscillation leakage is attributed to device offsets among the first and second digital-to-analog converters 301, 302, the first and second low-pass filters 303, 304, and the first and second transconductance stages 305, 306. As for radio frequency local oscillation leakage, it arises as a result of direct coupling due to parasitic capacitance or mutual inductance. Reductions of these two different types of local oscillation leakage need to be conducted independently.
It is noted that the article by C. Lee et al. does not disclose how to adjust the first and second transconductance stages 305, 306 and the phase and amplitude of the first and second digital base band signals (BBIt), (BBQt) for reducing local oscillation leakage and I/Q mismatch of a transmitter. Neither does the article mention how to reduce local oscillation leakage and I/Q mismatch of a receiver.